Amazon on Thursday estimated operating profit in its second quarter of $220 million to $320 million on revenue of $6.1 billion to $6.7 billion.

Analysts, on average, have been expecting an operating profit of $327.8 million on revenue of $6.43 billion, according to Thomson Reuters I/B/E/S.

The fact that operating income (outlook) has a much lower range to it is the key driver of the downside here after the close, said Scott Tilghman, analyst at Hudson Square Research.

In this case what we have is a very solid first quarter, but it looks like margins compressed ... so that might be a concern going through the rest of the year, he said.

BWS Financial analyst Hamed Khorsand noted that Amazon's forecast for second-quarter sales was slightly above what Wall Street had been expecting, so the lower operating profit view suggested the company was making a bigger investment in its business.

Amazon reported net income for the first quarter of $299 million, or 66 cents per share, up from $177 million, or 41 cents per share, a year earlier. Analysts, on average, had been expecting earnings of 61 cents per share.

Amazon's media segment, which includes books, has been improving, as it strives to offer the lowest prices against fierce competition from rivals such as Wal-Mart Stores Inc's online unit. That segment rose 26 percent in the first quarter.

Currency exchange rates also boosted revenue, the company said. Revenue rose 47 percent in North America and 45 percent internationally.

Amazon shares fell to $139.70 in extended trade after closing at $150.09 on Nasdaq.